NATS, Britain's national air traffic body, said a computer problem at its center in Swanwick, England, had touched off troubles in the system but that was fixed and the agency was in the process of returning to normal operations. Officials at London Heathrow Airport blamed the failures on a power outage.
"We apologize for any delays and the inconvenience this may have caused," NATS said in a statement.
The shutdown came at the start of the weekend in a sprawling city with five commercial airports, including London Heathrow — Europe's busiest — as well as Gatwick, Luton, Stansted and London City Airports.
Heathrow alone handles about 1,200 to 1,400 flights a day, about 200 of those to and from the United States, according to the flight tracking service FlightAware. As of 11 a.m. ET (1400 GMT), FlightAware was noticing some delays at Heathrow but few cancellations.
"The problem is mostly impacting departures at this point. It is late afternoon in the U.K. and the majority of US-bound flights depart in the morning," said Daniel Baker of FlightAware.
The Swanwick center has been plagued by problems since it opened more than a decade ago. It only swung into operation in 2002 — six years after its planned commissioning date and double its original budget. Software and reliability issues have caused repeated disruption since.
Last December, a computer problem at Swanwick took 12 hours to fix.
At Heathrow, passengers on United flight 941 to Newark were told about the computer problem as they boarded at 1530 GMT (10:30 am EST). United gate staff said the plane would be loaded with passengers and head toward a runway in the hopes that the computer problem could be fixed.
Most passengers at Heathrow seemed unaware of the issue and no further mention was made during normal boarding.
A Heathrow statement said "flights are currently experiencing delays and we will update passengers as soon as we have more information."
Airline security analyst Chris Yates said planes on the ground will not take off if the airspace was closed as they have no air traffic control cover. Those in the air that are close enough to the airport to have been in a holding pattern — say within 30 minutes of landing — will be allowed to land, he said.
Those further out will have to be diverted to other airports. Manchester Airport is open and accepting flights.
Yates said such computer issues happen rarely. One that occurred about 2 1/2 years ago resulted in disruptions that lasted for days.
A few months after the opening of the Swanwick center, the BBC cited an unnamed air traffic controller as complaining of "potentially catastrophic" problems at the facility, including erratic radio transmissions. Other controllers cited by the broadcaster said the text on their computer screens was "too tiny to read."
In 2004, Swanwick was in the news again when a computer problem grounded scores of flights across Britain. An even more serious glitch in September 2008 grounded hundreds of flights and affected tens of thousands of travelers.
The 2013 failure at Swanwick was particularly bad because it came at the beginning of the holiday season. After calm was restored, NATS chief Richard Deakin called that failure a "one-in-10-year event."
Raphael Satter and Kathleen Carroll in London and Joan Lowy in Washington, D.C., contributed.
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